build(docs): markdown for api descriptions

[devversion]

• Parses Class, Property and Method descriptions using a markdown filter from Dgeni.

Important: Markdown adds <p> elements around text and those have margins by default. The styles on material.angular.io would need to be revised too.

But still a big benefit because backticks are rendered as code snippets. Same would happen if there would be multi line code-snippets.

Closes #3924

[jelbourn]

LGTM

Feel free to send a corresponding PR to the docs repo for the css if you want

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